The present invention relates to continuous, reinforced, geometrically symmetrical, microporous membranes having three distinct pore zones and to processes of making and using same, more particularly to reinforced microporous membranes including a scrim having two sides at least substantially encapsulated within a first dope and at least one additional dope coated onto each side of the encapsulated scrim prior to the first dope being quenched and, most particularly, to a geometrically symmetric, continuous, reinforced membrane having three distinct pore zones including a scrim at least substantially and preferably, completely encapsulated by a relatively large pore size middle zone and two outer zones, one on each side of the middle zone, at least one of the three zones having a pore size at least about twenty (20%) percent greater than the other zones.
Microporous phase inversion membranes are well known in the art. Microporous phase inversion membranes are porous solids which contain microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels or paths through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous phase inversion membrane become trapped on or in the membrane structure effecting filtration. A slight pressure, generally in the range of about two (2) to about fifty (50) psid (pounds per square inch differential) is used to force fluid through the microporous phase inversion membrane. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores and some particles that are smaller than the pores are also trapped or absorbed into the membrane pore structure within the pore tortuous path. The liquid and some particles smaller than the pores of the membrane pass through. Thus, a microporous phase inversion membrane prevents particles of a certain size or larger from passing through it, while at the same time permitting liquid and some particles smaller than that certain size to pass through. Microporous phase inversion membranes have the ability to retain particles in the size range of from about 0.01 or smaller to about 10.0 microns or larger.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are about eight (8) microns in diameter, platelets are about two (2) microns in diameter and bacteria and yeast are about 0.5 microns or smaller in diameter. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry. Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out of a fully wetted phase inversion membrane (the initial Bubble Point, or "IBP"), and the higher pressure which forces air out of the majority of pores all over the phase inversion membrane (foam-all-over-point or "FAOP"). The procedures for conducting initial bubble point and FAOP tests are discussed in U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, the disclosure of which is herein incorporated by reference. The procedure for the initial bubble point test and the more common Mean Flow Pore tests are explained in detail, for example, in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference. The bubble point values for microporous phase inversion membranes are generally in the range of about two (2) to about one hundred (100) psig, depending on the pore size and the wetting fluid.
U.S. Pat. No. 3,876,738, the disclosure of which is herein incorporated by reference, describes a process for preparing microporous membranes by quenching a solution of a film-forming polymer in a non-solvent system for the polymer. U.S. Pat. No. 4,340,479, the disclosure of which is herein incorporated by reference, generally describes the preparation of skinless microporous polyamide membranes by casting a polyamide resin solution onto a substrate and quenching the resulting thin film of polyamide.
Since the mechanical strength of some microporous membranes is poor, it is known to reinforce such membranes with a porous support material to improve mechanical properties and facilitate handling and processing. Accordingly, the aforementioned U.S. Pat. No. 4,340,479 describes a procedure wherein a polymer solution is directly cast onto a porous support material so that the polymer solution penetrates the support material during casting and becomes firmly adhered thereto during formation of the reinforced inner layer of a composite microporous membrane. The support material preferably possesses an open structure so that pressure drop across the composite membrane is minimized. U.S. Pat. No. 4,340,479 further discloses combining two or more microporous membranes, one of which may be reinforced, to form a dual or triple layered structure which is dried under conditions of restraint to produce a single sheet having particle removal characteristics superior to those of individual layers.
U.S. Pat. No. 4,707,265, the disclosure of which is herein incorporated by reference, discloses a reinforced laminated filtration membrane comprising a porous reinforcing web impregnated with a polymeric microporous inner membrane and at least one polymeric microporous outer qualifying membrane laminated to each side of the impregnated web. The pore size of the inner membrane is greater than the pore size of the outer membranes. In this manner, the imperfections, e.g., fiber bundles, broken fibers, void areas, and the like, which are invariably present in the reinforcing web are confined to a coarse, more open inner membrane and the tighter outer qualifying layers are strengthened and supported by the web. The qualifying layers are not affected by imperfections present within the reinforcing web. Further, the use of a coarse, large pore size inner membrane layer insures that there is no substantial pressure drop of fluid across the reinforcing web.
The membranes disclosed in U.S. Pat. No. 4,707,265 are complicated and costly to produce since three separate operations are required to produce the composite membrane: first, the impregnated reinforced membrane support layer is produced, second, the non-reinforced qualifying layers are produced and, third, the impregnated reinforced membrane support layer and the non-reinforced qualifying layers are laminated to form the multilayer composite microporous membrane.
Due to processing and handling restraints, there is a limit to how thin the impregnated reinforced membrane support layer and the non-reinforced qualifying layers can be. As a result, the multilayer composite microporous membrane of U.S. Pat. No. 4,707,265 is at least about ten (10) mils thick. Furthermore, the overall pore size of the composite membrane described in U.S. Pat. No. 4,707,265 is generally limited to the range of approximately 0.45 microns or lower due to the difficulties of separately producing and handling non-reinforced qualifying layers having pore sizes of as high as about 0.45 micron. Thus, the utility of the laminated composite membrane is limited to sterilizing applications and other applications where membranes having about 0.65, 0.8, 1.2, 3.0 and greater micron ratings are not needed.
As the thickness of a membrane increases, pressure drop increases, flow rate worsens and the performance characteristics of the membrane are adversely affected. For example, with increasing thickness the total number of pleats in a pleated cartridge element decreases, thereby reducing the effective surface area available for filtration. Furthermore, a mechanical strain exists at the crest of each pleat and increases with increasing thickness. As a result, thick membranes are more likely to crack during the pleating, edge-seaming, etc. operations that are attendant to the production of pleated filter cartridge elements or during oxidative hydrolytic exposure or multiple steam cycling. Therefore, mechanical strains, which can never be fully relieved after cartridge fabrication, may decrease the useful life of the product and may lead to early failure in integrity.
U.S. Pat. No. 4,770,777 overcomes some of the shortcomings of the process disclosed in U.S. Pat. No. 4,707,265 by completely saturating the reinforcing web with a large pore size (coarser) membrane casting solution, applying a small pore size membrane casting solution on one side of the coated web and then quenching the large and small pore size casting solutions from only one side to provide a continuous, geometrically asymmetric membrane possessing a pore size gradient. Thus, the lamination step of U.S. Pat. No. 4,707,265 is eliminated, along with the necessity of handling the fragile non-reinforced qualifying layers. Further, following the teachings of this patent, it is not possible to apply another casting solution on the other side of the large pore size reinforced web containing layer. Thus, the only additional layers can be cast on top of the second layer cast on the first layer that includes the woven material. Additionally, the membrane taught in U.S. Pat. No. 4,770,777 is a skinned membrane. Accordingly, such membrane suffers from drawbacks associated with skinned microporous membranes, in particular, high pressure drop, poor structural integrity, susceptibility to skin breach, propensity to becoming fouled by debris, etc.
U.S. Pat. No. 5,433,859 attempts to address some of the deficiencies, in particular, high pressure drop, of the skinned membrane disclosed in U.S. Pat. No. 4,770,777 by proposing, preferably, an incomplete impregnation of the reinforcing web with coarse membrane casting solution so that a portion of the reinforcing web having a thickness of about 50 microns is not embedded within the microporous membrane. The low flow resistance of that portion of the reinforcing web which is not embedded within the microporous membrane ensures that filtered fluid passing through the supported microporous membrane will not have a significant adverse impact on the pressure drop across the filtration element.
While the membrane disclosed in U.S. Pat. No. 5,433,859 exhibits lower pressure drop across the membrane compared to the skinned membrane disclosed in U.S. Pat. No. 4,770,777, the membrane does have significant structural drawbacks. First, the membrane suffers from tremendous geometric asymmetry around the central axis of the reinforcing web, i.e., the thickness of the membrane varies on each side of the reinforcing web. As a result, when the membrane is pleated, the mechanical strain on the thick side of the membrane is greater than on the thin side of the membrane. This differential in mechanical strain increases the possibility of stress crack formation and failure of the integrity of the membrane. Second, the membrane poses a possible risk of separation along the membrane-reinforcing web interface, especially during backwashing operations. Third, the membrane exhibits "sidedness" having a different pore size on one side versus the other side and an exposed scrim reinforcement area. This will limit its utility in certain applications such as analytical, or some diagnostic filtration techniques. Finally, as with the U.S. Pat. No. 4,720,777, the membrane of the U.S. Pat. No. 5,433,859 cannot have another section on the opposite side of the membrane-reinforced web for the same reason as the U.S. Pat. No. 4,770,777.
Thus, there is a need for a relatively thin geometrically symmetrical, continuous, monolithic, reinforced, polymeric microfiltration membrane having at least three independent and distinct pore size performance zones (one reinforced performance zone, presently preferably, central to the membrane structure, and two outer non-reinforced performance zones including at least one outer qualifying performance zone on one side of the central reinforced zone and a second outer non-qualifying prefilter performance zone on the other side of the central performance zone or, two outer qualifying performance zones, one on each side of the central zone) progressing through the thickness of the membrane, each zone being continuously joined throughout the membrane structure. The three zones should be continuously joined by the molecular entanglement which occurs in the liquid state of the dope after the dope of each outer zone is coated onto the dope of the central zone prior to quenching and not by a lamination bond after quenching. Such a three zone membrane structure should be produced by a highly robust, single unit operation, with on-line pore size and layer thickness attribute control. Such a three zone membrane should meet the industries long recognized need for superior performance and greater flexibility of triple layer composite structures. Such a three zone membrane should be relatively inexpensively and easily manufactured. Such a three zone membrane should simplify the production of traditional laminated single layer structure membrane and increase the range of pore sizes and manageable handling thickness that are provided by the non-reinforced zones. Such a three zone membrane should avoid the highly geometrically asymmetric structure of the two zone prior art membrane. Such a three zone membrane should have a geometrically symmetric structure having improved utility, flexibility, and processability into finished industrial forms (pleated cartridges, etc.) while assuring structural integrity. Such a three zone membrane should possess a surprisingly thin cross section, having three independent performance zones in a geometrically symmetrical, continuous, monolithic, reinforced, polymeric, microfiltration membrane. Such a three zone membrane should have a robust mechanical strength, suitable for pleating and industrial handling and capable of being produced on-line and in real time in a surprisingly wide range of pore size attributes, when the apparatus of the present application is coupled with the commonly assigned copending application Ser. No. 09/022,295, filed Feb. 11, 1998, now U.S. Pat. No. 6,056,529, entitled "Methods and Systems For Producing A Plurality of Different Microporous Phase Inversion Membrane Each Having Any One Of A Plurality Of Different Pore Sizes from a Single Master Dope Batch", the disclosure of which is herein incorporated by reference. Such a three zone membrane should have a minimum functional thickness providing maximum throughput at minimal pressure drops, high integrity and be economically produced in a single manufacturing operation.